Using setInterval() to do simplistic continuous polling

A simple non-blocking poll function can be implemented in recent browsers using Promises:

var sleep = duration => new Promise(resolve => setTimeout(resolve, duration))
var poll = (promiseFn, duration) => promiseFn().then(
             sleep(duration).then(() => poll(promiseFn, duration)))

// Greet the World every second
poll(() => new Promise(() => console.log('Hello World!')), 1000)

From my comment:

I would use setTimeout ^[docs] and always call it when the previous response was received. This way you avoid possible congestion or function stacking or whatever you want to call it, in case a request/response takes longer than your interval.

So something like this:

function refresh() {
    // make Ajax call here, inside the callback call:
    setTimeout(refresh, 5000);
    // ...
}

// initial call, or just call refresh directly
setTimeout(refresh, 5000);